Exhaust gas cleaner for internal combustion engine

ABSTRACT

In an internal combustion engine ( 100 ) provided with an NO X  absorbing reduction catalyst ( 2 ) in an exhaust gas passage ( 1 ), there are arranged: an oxygen sensor ( 3 ) which is mounted downstream of the NO X  absorbing reduction catalyst ( 2 ) in the exhaust gas passage ( 1 ); and a determination means for determining a condition of degradation of the NO X  absorbing reduction catalyst ( 2 ) on the basis of a time length, during which a voltage value, having a small amount of variation before the voltage value outputted from the oxygen sensor ( 3 ) is recorded as a maximum value when executing a rich spike, is recorded.

TECHNICAL FIELD

[0001] The present invention relates to an exhaust gas cleanup devicefor an internal combustion engine provided with a NOx absorbingreduction catalyst which locates in an exhaust gas passage.

BACKGROUND ART

[0002] In an internal combustion engine provided with a NOx absorbingreduction catalyst (hereinafter, referred to as absorbing catalyst)which locates in an exhaust gas passage, when NOx is absorbed by theabsorbing catalyst up to a certain level, the NOx thus absorbed isreduced and removed by executing an operation called a rich spike bywhich an air-fuel ratio is switched temporarily and rapidly fromleanness to richness.

[0003] As an invention of method for recovering the absorbing catalystby the rich spike, there is a Japanese Laid-Open Patent Publication No.2000-45752 (Method for Cleaning NOx Absorbing Reduction Catalyst inInternal Combustion Engine), the applicant of which is the same of thepresent application. In the invention of the Japanese Laid-Open PatentPublication No. 2000-45752, a consideration is given to a point that anabsorbing ability of the absorbing catalyst is fully realized by takingthe following two steps. As a first step, a possible absorbing capacityof NOx by the absorbing catalyst is monitored. As a second step, when(or before) the accumulated value of the NOx flowing in the absorbingcatalyst reaches a possible absorbing capacity, the operation of therich spike is executed to clean the absorbing catalyst.

[0004] The absorbing catalyst is, however, poisoned by a sulfuriccomponent included in the exhaust gas, and the absorbing catalystdeteriorates with time passing so that the possible absorbing capacityof NOx decreases. As a method for removing the sulfuric component fromthe poisoned absorbing catalyst, there is a Japanese Laid-Open PatentPublication No. 2000-8909 (Method for Controlling Internal CombustionEngine), the applicant of which is the same of the present application.In the invention of the Japanese Laid-Open Patent Publication No.2000-8909, when a predetermined time (100 minutes, for example) elapseswhile the internal combustion engine is operating, the air-fuel ratio isswitched over from the leanness to the richness, and the absorbingcatalyst is recovered by operating it during a predetermined time(approximately 100 minutes) under a condition that the temperature ofthe exhaust gas is more than 600° C.

[0005] In this way, according to the conventional art, to which degreethe absorbing catalyst is poisoned by the sulfuric component isestimated only from the operation time of the internal combustionengine. That is, the recovering work is performed mechanically when thepredetermined time passes, without monitoring the actual deterioratingcondition of the absorbing catalyst. The “deterioration of the absorbingcatalyst” means decrease of performance of cleanup of the absorbingcatalyst on the basis of the poisoning by the sulfuric component and byheat.

DISCLOSURE OF INVENTION

[0006] (Technical Object to Be Achieved by Invention)

[0007] The technical object is directed toward a provision of an exhaustgas cleanup device for an internal combustion engine, in which acondition of actual deterioration of the absorbing catalyst ismonitored, and in which it is possible to recover the absorbing catalystin a short time.

[0008] (How to Achieve the Technical Object)

[0009] According to the invention of claim 1, in order to achieve theaforementioned technical object, in an exhaust gas cleanup device of aninternal combustion engine which is provided with an NO_(X) absorbingreduction catalyst in an exhaust gas passage, there are provided: anoxygen sensor which is mounted downstream of the NO_(X) absorbingreduction catalyst in the exhaust gas passage; and a determination meansfor determining a condition of deterioration of the NO_(X) absorbingreduction catalyst on a basis of a time length, during which a voltagevalue, having a small amount of variation before the voltage valueoutputted from the oxygen sensor is recorded as a maximum value when arich spike is executed, is recorded.

[0010] According to the invention of claim 2, in the invention of claim1, there is provided an air-fuel ratio setting means for setting densityof CO so as to increase the density of CO inside the exhaust gas passagewhich is upstream of the NO_(X) absorbing reduction catalyst at a timeof recovering the NO_(X) absorbing reduction catalyst, as degree of thedeterioration, determined by the determination means, of the NO_(X)absorbing reduction catalyst becomes higher.

[0011] According to the invention of claim 3, in the invention of claim2, an air-fuel ratio in the exhaust gas passage which is upstream of theNO_(X) absorbing reduction catalyst is set so that the density of CO inthe exhaust gas passage which is downstream of the NO_(X) absorbingreduction catalyst is kept constant, at the time of recovering theNO_(X) absorbing reduction catalyst.

[0012] According to the invention of claim 4, in the invention of one ofclaims 1-3, there are provided an exhaust gas flow rate detecting means;an NO_(X) density detecting means for detecting density of NO_(X) in anexhaust gas; a temperature sensor for detecting a temperature of theNO_(X) absorbing reduction catalyst; and a calculating means forcalculating amount of NO_(X) flowing in the NO_(X) absorbing reductioncatalyst per unit time, from an exhaust gas flow rate detected by theexhaust gas flow rate detecting means and from an NO_(X) densitydetected by the NO_(X) density detecting means, wherein a possibleNO_(X) absorbing capacity of the NO_(X) absorbing reduction catalyst isestimated by the temperature sensor, and wherein the NO_(X) absorbingreduction catalyst is recovered when an accumulated amount of NO_(X)flowing in the NO_(X) absorbing reduction catalyst reaches a possibleabsorbing amount.

[0013] According to the invention of claim 5, in the invention of claim4, the possible NO_(X) absorbing capacity of the NO_(X) absorbingreduction catalyst which is deteriorated, is estimated, and an intervalfor executing the rich spike is set in compliance with the possibleNO_(X) absorbing capacity.

[0014] According to the invention of claim 6, in the invention of one ofclaims 2 and 3, there are provided a secondary air supply passage and anoxidation catalyst, both of which are in the exhaust gas passagedownstream of the NO_(X) absorbing reduction catalyst, wherein CO havingpassed through the NO_(X) absorbing reduction catalyst is oxidized inthe oxidation catalyst.

[0015] According to the invention of claim 7, in the invention of claim1, there is provided a three way catalyst which is in the exhaust gaspassage upstream of the NO_(X) absorbing reduction catalyst, wherein thethree way catalyst has an oxygen absorbing function and an oxidizingfunction.

[0016] According to the invention of claim 8, in the invention of claim7, the three way catalyst and the NO_(X) absorbing reduction catalystare unitized integrally, and the three way catalyst is upstream of theexhaust gas passage.

[0017] (More Advantageous Effect Than Prior Art)

[0018] According to the invention of claim 1, the CPU 4 compares thevoltage data outputted from the oxygen sensor 3 in which the voltagedata are about the absorbing reduction catalyst 2 that is notdeteriorated and in which the voltage data are stored on the memory 5,and the waveform of the value, actually measured, of voltage outputtedfrom the oxygen sensor 3 at the time of executing the rich spike, toeach other. On the basis of the comparison, the degree of course ofdeterioration of the absorbing reduction catalyst 2 can be estimated.Therefore, it is possible to keep the absorbing reduction catalyst 2 ina condition for exerting the NO_(X) absorbing function fully, and theNO_(X) can be purged in good condition.

[0019] According to the invention of claim 2, there is provided theair-fuel ratio setting means (fuel supply amount adjustment valve) forsetting the density of CO inside the exhaust gas passage (exhaust gaspipe 1) which is upstream of the NO_(X) absorbing catalyst 2, incompliance with the degree of deterioration of the absorbing catalyst 2;therefore, the NO_(X) absorbing catalyst 2 can be recovered in goodcondition. Consequently, always, the NO_(X) can be purged in goodcondition.

[0020] According to the invention of claim 3, in the invention of claim2, the air-fuel ratio λ which is upstream of the NO_(X) absorbingreduction catalyst 2 is set so that the density of CO which isdownstream of the NO_(X) absorbing reduction catalyst 2 is keptconstant. Therefore, it is possible to make the time for recovering theabsorbing catalyst 2 the shortest while the density of discharged CO issuppressed to a predetermined density within a value of an environmentallimit. Consequently, it is possible to maintain the thermal efficiencyhigh.

[0021] According to the invention of claim 4, the possible NO_(X)absorbing capacity of the absorbing catalyst 2 is estimated, and theabsorbing catalyst 2 is recovered when the total amount of NO_(X)(accumulated amount of NO_(X)) flowing in the absorbing catalyst 2reaches the possible NO_(X) absorbing capacity. Therefore, it ispossible to exert the absorbing ability of the absorbing catalyst 2 tothe fullest extent.

[0022] Regardless of change in operational circumstances such as enginespeed, engine load, etc., the total amount of the discharged NO_(X) iscalculated. Therefore, it is possible to properly determine the time torecover the absorbing catalyst 2, and the good absorbing ability can beexerted.

[0023] According to the invention of claim 5, the possible NO_(X)absorbing capacity is estimated in compliance with the degree ofdeterioration of the absorbing catalyst 2, and the interval forperforming the rich spike is set up. Therefore, the minimum necessaryrecovery can be performed in compliance with the degree of deteriorationof the absorbing catalyst 2, the operation time can be the shortest withthe air-fuel ratio λ being rich, the discharge of CO can be mademinimum, and the heat efficiency can be maintained high.

[0024] According to the invention of claim 6, there is arranged theoxidation catalyst 20 which is in the exhaust gas passage (exhaust gaspipe 1) downstream of the absorbing catalyst 2. Therefore, the dischargeof CO which has not been employed upon recovery of the absorbingreduction catalyst 2, into the atmospheric air, is surely prevented. Inother words, even if a large amount of CO, effective in the recovery, isflowed, the CO (CO which has not been employed for recovery) havingpassed through the absorbing catalyst 2 can undergo an oxidizing processby the oxidation catalyst 20. Therefore, the discharge of CO into theair can be prevented.

[0025] According to the invention of claim 7, there is arranged thethree way catalyst 19 which is in the exhaust gas passage (exhaust gaspipe 1) upstream of the absorbing catalyst 2, and oxygen is absorbedupstream of the absorbing catalyst 2 by the three way catalyst 19 at thetime of recovery of the absorbing catalyst 2. Therefore, the absorbingcatalyst 2 can be recovered in good condition, and the good cleaningability can be realized.

[0026] According to the invention of claim 8, the three way catalyst 19and the absorbing catalyst 2 are unitized integrally, which are providedin the exhaust gas passage (exhaust gas pipe 1). Therefore, thetemperature of the exhaust gas between the three way catalyst 19 and theabsorbing catalyst 2, can be prevented from dropping down, and it ispossible to recover the absorbing catalyst 2 at a high temperature.Therefore, the absorbing catalyst 2 can be surely recovered, and theabsorbing catalyst 2 can make the NO_(X) be cleaned up in goodcondition.

BRIEF DESCRIPTION OF DRAWINGS

[0027]FIG. 1 is a schematic front view of an internal combustion enginewhich is embodied according to the invention of claim 1.

[0028]FIG. 2 is a graph showing a waveform of a voltage outputted froman oxygen sensor.

[0029]FIG. 3 is a graph showing a change in CO density upstream anddownstream of an absorbing catalyst when time elapses.

[0030]FIG. 4 is a graph showing a change in CO density upstream of theabsorbing catalyst, with the CO density downstream of the absorbingcatalyst being kept constant.

[0031]FIG. 5 is a graph showing a relation between temperature of theabsorbing catalyst and possible NOx absorbing capacity.

[0032]FIG. 6 is a schematic front view of the internal combustion enginewhich is embodied according to the invention of claims 7 and 8.

[0033]FIG. 7 is a schematic front view of the internal combustion enginewhich is embodied according to the invention of claim 4.

[0034]FIG. 8 is a graph showing a change in temperature of exhaust gaswith time passing, when a condition of operation of the internalcombustion engine of FIG. 7 changes.

[0035]FIG. 9 is a graph showing a temperature distribution of theabsorbing catalyst on the basis of the engine load and engine speed.

[0036]FIG. 10 is a graph showing a relation between recovering speed ofthe absorbing catalyst and air-fuel ratio λ.

[0037]FIG. 11 is a schematic front view of the internal combustionengine which is embodied according to the invention of claim 6.

[0038]FIG. 12 is a graph showing a relation between the recovering speedof the absorbing catalyst and the temperature.

[0039]FIG. 13 is a graph showing a relation between the air-fuel ratioλ, and CO and NOx densities downstream of the absorbing catalyst.

[0040]FIG. 14 is a graph showing a relation between a CO density setupstream of the absorbing catalyst and time when the recovery thereofhas been accomplished, in a case that the absorbing catalyst is notdeteriorated so much and in a case that the absorbing catalyst isdeteriorated appreciably.

BEST MODE FOR CARRYING OUT THE INVENTION

[0041] (Embodiment of Invention of Claim 1)

[0042]FIG. 1 is a schematic front view of an internal combustion engine100 into which the invention of claim 1 is embodied. The internalcombustion engine 100 has an exhaust gas pipe 1 with which an NOxabsorbing reduction catalyst 2 (hereinafter, referred to as an absorbingcatalyst 2) is unitized. In the exhaust gas passage (exhaust gas pipe 1)downstream of the absorbing catalyst 2, there is provided an oxygensensor 3. Also, in the exhaust gas passage (exhaust gas pipe 1) upstreamof the absorbing catalyst 2, there is provided an oxygen sensor 3 a forprecisely detecting the air-fuel ratio λ of the exhaust gas. Each of theoxygen sensors 3, 3 a is connected to a CPU 4 through signal cables 6, 6a, respectively. Signals detected by the oxygen sensors 3, 3 a, aretransmitted to the CPU 4 through the signal cables 6, 6 a. The CPU 4 isaccessible to a memory 5 which will be explained in detail below.

[0043]FIG. 2 is a graph showing a waveform of a voltage outputted fromthe oxygen sensor 3. The absorbing catalyst 2 shown in FIG. 1, absorbsNOx included in the exhaust gas flowing through the exhaust gas pipe 1.But the absorbing catalyst 2 can absorb no more NOx once the amount ofabsorption thereof reaches a possible absorbing capacity. When theamount of absorption reaches 90%, for example, of the possible absorbingcapacity, an operation called rich spike is executed in order to set theair-fuel ratio λ slightly on a side of richness with respect to atheoretical air-fuel ratio.

[0044] Referring to FIG. 2, the rich spike is performed from time t₁ totime t₂. When the internal combustion engine 100 (FIG. 1) operatesnormally, the air-fuel ratio λ is set to be lean (λ=1.3˜1.5). As thevalue of the air-fuel ratio λ is greater (namely, leaner), the densityof oxygen is higher. As the oxygen density is higher, the voltageoutputted from the oxygen sensor 3 decreases. Therefore, as shown inFIG. 2, since the oxygen density is low during the rich spike, thevoltage outputted therefrom is high.

[0045] In FIG. 2, the value of the voltage hardly changes with respectto the value of E_(A) during a time from t₃ to t₄ (“small amount ofvariation in voltage value” in claim 1), and the value thereof rapidlyincreases again after passing the time t₄. When the air-fuel ratio λ islean, the absorbing catalyst 2 absorbs NO_(X) and oxygen,simultaneously. When the rich spike is executed, the oxygen absorbed bythe absorbing catalyst 2 is released, and the oxygen density in theexhaust gas pipe 1 downstream of the absorbing catalyst 2 becomes hightemporarily. And, in spite of executing the rich spike until all theoxygen thus absorbed is released completely, the oxygen density hardlychanges during the time from t₃ to t₄, which is reflected upon the valueof voltage outputted from the oxygen sensor 3.

[0046] The rich spike is performed five seconds, for example. When theabsorbing catalyst 2 is new, the oxygen absorbed thereby at the time t₄is released completely. When the deterioration of the absorbing catalyst2 proceeds, the oxygen absorbed thereby at the time t₅ before reachingthe time t₄, is released completely, for example, as shown in FIG. 2.

[0047] When the absorbing catalyst 2 is new and it has a high ability toabsorb it, the absorbing catalyst 2 absorbs a lot of NO_(X) togetherwith oxygen. However, when the absorbing catalyst 2 is employed duringmany hours, and when the deterioration (poisoning by the sulfuriccomponent) of the absorbing catalyst 2 proceeds, the ability to absorbit decreases. As a result, the amount of oxygen released at the time ofexecuting the rich spike becomes small. Therefore, when the absorbingcatalyst 2 is deteriorated, all the oxygen at the time t₅ shown in FIG.2 is released completely, and the value of the voltage increases up tothe highest value E_(B) at a relatively early stage.

[0048] Consequently, it is possible to know the degree of course ofdeterioration of the absorbing catalyst 2, on the basis of the waveformof the voltage outputted from the oxygen sensor 3. Accordingly, thecorrelation between the oxygen absorbing amount (NO_(x) absorbingamount) of the NO_(X) absorbing catalyst 2 and the waveform of thevoltage, is gained by experiment in advance, and the correlationtherebetween is stored on the memory 5 in FIG. 1. By comparing datastored on the memory 5 and the waveform of the value of voltage,actually measured, which is outputted from the oxygen sensor 3 at thetime of executing the rich spike with the CPU 4, it is possible toestimate the degree of course of deterioration of the absorbing catalyst2.

[0049] (Embodiment of Invention of Claim 2)

[0050] As shown in FIG. 1, an air supply pipe 7 and a fuel supply pipe 9are connected to a mixer 8. A mixture formed in the mixer 8, is suppliedfrom the mixer 8 to a combustion chamber (not shown) of the internalcombustion engine 100 via a mixture supply pipe 11, and the mixture ismade to combust in the combustion chamber.

[0051] The air-fuel ratio λ of the mixture supplied to the combustionchamber, can be changed by adjusting the valve travel of a fuel supplyamount adjustment valve 10 which is provided intermediately in the fuelsupply pipe 9. Namely, when the degree of the valve travel is madesmall, the amount of supply of the fuel decreases, and therefore theair-fuel ratio λ becomes high (i.e. becomes lean). On the contrary, whenthe degree of the valve travel is made large, the air-fuel ratio λbecomes low (i.e. becomes rich). The change of the air-fuel λ, in thevicinity of λ=1, is detected by the oxygen sensor 3 a with highprecision.

[0052]FIG. 13 shows a graph indicating a relation amongst the air-fuelratio λ, CO density and NO_(X) density. As shown in FIG. 13, as theair-fuel ratio λ is richer, the density of CO downstream of theabsorbing catalyst is higher and the density of NO_(X) is lower. On thecontrary, as the air-fuel ratio λ is leaner, the density of COdownstream of the absorbing catalyst is lower and the density of NO_(X)is higher. The region in which both of the CO density and the NO_(X)density are relatively low, is called a cleanup window.

[0053] The work for removing the sulfuric component from a deterioratedabsorbing catalyst 2, is called a “recovery”. When the absorbingcatalyst 2 is recovered, the sulfuric component is removed from theabsorbing catalyst 2, and the possible NO_(X) absorbing capacity of theabsorbing catalyst 2 is closer to the possible NO_(X) absorbing capacitythereof when it is new.

[0054] As shown in FIG. 10, as the CO density is higher, the recoveryspeed is faster. The sulfuric component is advantageously removed withhigher CO density. Hence, at the time of recovery thereof, the valvetravel of the fuel supply amount adjustment valve (air-fuel ratiosetting means) 10 is adjusted, and the air-fuel ratio λ is set to be onan end (λ=0.99-0.997) of the rich side of the cleanup window. At thesame time, the internal combustion engine 100 is operated with such atemperature of the exhaust gas as allows the recovery. It is preferablethat the temperature at this time is more than 600° C.

[0055] As shown in FIG. 3, when the CO density upstream of the catalystis kept constant, the CO density downstream of the absorbing catalystbecomes low initially. But when the time t_(A) comes, the CO densitydownstream of the absorbing catalyst increases up to a predeterminedvalue. When a new absorbing catalyst having a large possible absorbingcapacity absorbs the NO_(X) and oxygen up to a limitation of absorption,the amount of oxygen and NO_(x) which is made to react with CO at thetime of reduction of the absorbing catalyst increases. Therefore, theredoes not exist any object (NO_(x) and oxygen) with which CO reacts, andit takes long before the CO density increases up to a predeterminedvalue. But the possible absorbing capacity of the absorbing catalystundergoing a considerable poisoning is small, and the amount of oxygenand NO_(x) having been absorbed is small. Therefore, the amount ofoxygen and NO_(x) which is made to react with CO at the time ofreduction of the absorbing catalyst is small, and the CO densityincreases up to the predetermined value when the time t_(A), prior tothe time t_(B), comes.

[0056] As shown in FIG. 4, when the CO density is set to be highinitially at the time of recovery thereof, not so much of CO flowsdownstream of the absorbing catalyst. Accordingly, by setting the COdensity upstream of the absorbing catalyst to be low in accordance withthe degree of the recovery, it is possible to shorten the time torecover the absorbing catalyst while suppressing the CO densitydownstream of the absorbing catalyst.

[0057] In FIG. 4, the CO density downstream of the absorbing catalyst isset to be within an environmental discharge limit, and the CO densityupstream of the absorbing catalyst is set so that the CO densitydownstream of the absorbing catalyst during the recovery of theabsorbing catalyst become the set value. By heightening the CO densityupstream of the absorbing catalyst in advance during the recovery sothat the CO density downstream of the absorbing catalyst does not exceedthe environmental limit, it is possible to shorten the time of therecovery of the absorbing catalyst with the density of CO in the exhaustgas being kept low.

[0058]FIG. 14 shows a graph indicating a relation between a CO densityset upstream of the absorbing catalyst and a time when the recovery isaccomplished, in a case that the absorbing catalyst undergoes not somuch deterioration and in a case that the absorbing catalyst undergoesconsiderable deterioration. As shown in FIG. 14, when the setting COdensity is the same, the amount of poisoning increases as thedeterioration thereof proceeds. Therefore, it takes longer before therecovery is finished. Also, making the recovery time the same, it ispossible to set the CO density low at the time of recovery in a casethat it undergoes less deterioration.

[0059] (Embodiment of Invention of Claim 3)

[0060] In the embodiment of the invention of claim 2, the air-fuel ratioλ upstream of the absorbing catalyst 2 is set so that the density of COin the exhaust gas pipe 1 downstream of the absorbing catalyst 2 remainsconstant while the absorbing catalyst 2 is being recovered.

[0061] The deterioration of the absorbing catalyst 2 is patterned on thebasis of the degree of deterioration, a map is made by investigating arelation between the density of Co set downstream and the density of COadjusted upstream, in advance, and the map is stored on the memory 5.

[0062] The CPU 4 adjusts the valve travel of the fuel supply amountadjustment valve 10 so as to be able to properly adjust the density ofCO upstream with reference to the map thus stored on the memory 5, byestimating the degree of the deterioration of the absorbing catalyst 2from the waveform (FIG. 2) of the voltage outputted from the oxygensensor 3, and by selecting a setting value of the density of Codownstream.

[0063] (Embodiment of Invention of Claim 4)

[0064] As shown in FIG. 7, the internal combustion engine 102 has anengine speed detector 13 and an engine load detector 14. The detectionsignals having been detected thereby, are supplied to the CPU 4. Also,the exhaust gas pipe 1 has a temperature sensor 15. The CPU 4 estimatesthe temperature of the absorbing catalyst 2 from the temperature,detected by the temperature sensor 15, of exhaust gas.

[0065]FIG. 5 shows a graph indicating a relation between the temperatureof the absorbing catalyst 2 and the NO_(X) possible absorbing capacity.As shown in FIG. 5, the possible NO_(X) absorbing capacity varies whenthe temperature becomes high, no matter whether the absorbing catalyst 2is new or deteriorated. Therefore, it is possible to calculate thepossible absorbing capacity thereof, from the degree of deteriorationand temperature of the absorbing catalyst 2.

[0066] Firstly, the relation between the temperature and the possibleabsorbing capacity per degree of deterioration of the absorbing catalyst2, is gained by experiment in advance, and then the map is made andstored on the memory 5. The temperature of the absorbing catalyst 2changes in accordance with a state of operation of the internalcombustion engine 102. The temperature thereof is detected by thetemperature sensor 15, and the detection signal(s) detected therebyis/are transmitted to the CPU 4. Incidentally, as shown in FIG. 9, it isunderstood that the temperature of the exhaust gas (temperature of theabsorbing catalyst 2) increases in each of the case that the engine loadincreases and the case that the engine speed increases.

[0067] The degree of deterioration of the absorbing catalyst 2 can beestimated from the waveform of the voltage outputted by the oxygensensor 3 shown in FIG. 2. Consequently, the possible NO_(X) absorbingamount at present by the absorbing catalyst 2 can be gained on the basisthereof.

[0068] Next, how much density the NO_(X) which flows in the absorbingcatalyst 2 has, is checked. It is possible to estimate the condition ofoperation of the internal combustion engine 100 from the air-fuel ratioλ, the engine speed detected by the engine speed detector 13, and theengine load detected by the engine load detector 14. On the basis ofthese, it is possible to detect the exhaust gas flow rate and thedensity of NO_(X) included in the exhaust gas (exhaust gas flow ratedetection means and NO_(X) density detection means).

[0069] The amount per unit time of NO_(X) which flows in the absorbingcatalyst 2, is calculated by the CPU 4 (NO_(X) amount calculationmeans). The CPU 4 performs the rich spike, when the amount thereofreaches 90-95%, for example, of the present possible absorbing capacity,calculated above, of the absorbing catalyst 2; and the NO_(X) havingbeen absorbed is reduced and removed. In this way, it is possible tobring out a full ability of the absorption in accordance with thedeterioration of the absorbing catalyst 2, and possible to clean up theexhaust gas in good condition.

[0070] Of course, it is possible to absorb it until the total amount(accumulated NO_(X) amount) of NO_(X) calculated by the CPU 4 reaches100% of the possible absorbing capacity of the absorbing catalyst 2, andit is possible to perform the rich spike thereafter. However, there is apossibility that the amount of NO_(X) in the exhaust gas discharged tothe atmospheric air increases. Therefore, it is preferable to set about90-95% of the possible absorbing capacity as an upper limit, asaforementioned.

[0071] (Embodiment of Invention of Claim 5)

[0072] In the embodiment of claim 4, about 90-95% of the possibleabsorbing capacity of the absorbing catalyst 2 is set as the upperlimit. At the time of reducing and removing the NO_(X) having beenabsorbed, the rich spike is performed during the time which is necessaryfor reducing and removing the NO_(X) the amount of which corresponds tothe estimated amount of NO_(X) computed by the CPU 4.

[0073] That is, the rich spike is performed after the lean operationtime during which the amount of NO_(X) (or about 90-95% of the possibleabsorbing capacity), corresponding to the possible absorbing capacity ofthe absorbing catalyst 2, is absorbed, elapses.

[0074] When the rich spike is executed in this way, it is possible tocleanup the absorbing catalyst in good condition, to suppress to arequisite minimum the amount of Co discharged when the air-fuel ratio λis rich, and to suppress the decrease of the thermal efficiency to aminimum.

[0075] (Embodiment of Invention of Claim 6)

[0076]FIG. 11 is a schematic front view of the internal combustionengine 104 which is embodied in accordance with the invention of claim6. According to the invention of the aforementioned claims 2 and 3, theair-fuel ratio λ at the time of recovering the absorbing catalyst 2, isset so that the density of CO in the exhaust gas pipe 1 upstream of theabsorbing catalyst 2 is higher. But it is necessary to take a countermeasure for preventing CO, which does not contribute to the cleanup ofthe NO_(X) and SO_(X) in the exhaust gas, from being discharged into theatmospheric air. Consequently, the internal combustion engine 104 has anarrangement in which an oxidation catalyst 20 is provided in the exhaustgas passage (exhaust gas pipe 1) downstream of the absorbing catalyst 2.

[0077] Further, the internal combustion engine 104 has a pump 21, forsupplying the secondary air, which is mounted in the exhaust passageupstream of the oxidation catalyst 20. There is installed an oxygensensor 3 b between the pump 21 and the absorbing catalyst 2. The oxygensensor 3 b is necessarily mounted upstream of the pump 21 in order notto allow the oxygen sensor 3 b to detect oxygen in the secondary air.The oxygen sensor 3 b detects only the oxygen passing through theabsorbing catalyst 2, and the oxygen sensor 3 b fulfills a role formonitoring the absorbing capacity of the absorbing catalyst 2. Otherconstructions of the internal combustion engine 104 are the same asthose of the internal combustion engine 100. Air supplied by the pump 21oxidizes (i.e. cleans up) CO in the oxidation catalyst 20.

[0078] Incidentally, in the invention of claims 2 and 3, it is necessaryto prevent a discharge of CO, which is excessively supplied, into theatmospheric air. Namely, in order to limit the amount of NO_(X) or COdischarged to the atmospheric air to a minimum, it is necessary torelatively strictly control the air-fuel ratio.

[0079] In this respect, according to the invention of claim 6, thecleanup by the oxidation catalyst 20 can be done even if a bit excessiveamount of CO is supplied, and it is easy to control the air-fuel ratio.Namely, in the internal combustion engine 104, the oxygen sensor 3 a canbe skipped.

[0080] (Embodiment of Invention of Claims 7 and 8)

[0081]FIG. 6 is a schematic front view of the internal combustion engine103 which is embodied in accordance with the invention of claims 7 and8. The internal combustion engine 103 differs from the internalcombustion engine 100, only in that a three way catalyst 19 is arrangedupstream of the absorbing catalyst 2. However, the other constructionsof the internal combustion engine 103 are the same as those of theinternal combustion engine 100. Upon control of the air-fuel ratio λ,the fuel supply amount adjustment valve 10 is operated until the valueof voltage outputted from the oxygen sensor 3 a rapidly changes, and theair-ratio λ prior to the operation of the fuel supply amount adjustmentvalve 10 is detected on the basis of the operation amount of the fuelsupply amount adjustment valve 10. This operation is called a leanspike. Upon the recovery of the absorbing catalyst 2, firstly, the leanspike is executed. After detecting what value the present air-fuel ratioλ is, the air-fuel ratio λ is shifted toward a rich side by an amountnecessary to recover it.

[0082] When there exists oxygen, the recovery of the absorbing catalyst2 is blocked. Therefore, it is necessary to remove oxygen generated atthe time of execution of the lean spike, before the recovery work isstarted. Namely, the oxygen is absorbed by the three way catalyst 19mounted upstream of the absorbing catalyst 2, and the oxygen isprevented from flowing to the absorbing catalyst 2 downstream.

[0083] As shown in FIG. 6, by unitizing the three way catalyst 19 andthe absorbing catalyst 2, and by mounting the unit in the exhaust gaspassage (exhaust gas pipe 1), the temperature of the exhaust gas betweenthe three way catalyst 19 and the absorbing catalyst 2 is prevented fromdropping down.

[0084]FIG. 12 is a graph showing a relation between the recovery speedand temperature of the absorbing catalyst 2. As shown in FIG. 12, therecovery speed of the absorbing catalyst 2 is higher as the temperatureis higher. Therefore, in the internal combustion engine 103, theabsorbing catalyst 2 can be recovered in a short time by the exhaust gasat a high temperature.

INDUSTRIAL APPLICABILITY

[0085] The present invention is applicable to internal combustion landand marine engines which are equipped with NO_(X) absorbing reductioncatalysts in the exhaust gas passages.

1. An exhaust gas cleanup device of an internal combustion engine (100)which is provided with an NO_(X) absorbing reduction catalyst (2) in anexhaust gas passage (1), characterized in that there are provided: anoxygen sensor (3) which is mounted downstream of the NO_(X) absorbingreduction catalyst (2) in the exhaust gas passage (1); and adetermination means for determining a condition of deterioration of theNO_(X) absorbing reduction catalyst (2) on a basis of a time length,during which a voltage value, having a small amount of variation beforethe voltage value outputted from the oxygen sensor (3) is recorded as amaximum value when a rich spike is executed, is recorded.
 2. The exhaustgas cleanup device of the internal combustion engine as claimed in claim1, which comprises an air-fuel ratio setting means for setting densityof CO so as to increase the density of CO inside the exhaust gas passage(1) which is upstream of the NO_(X) absorbing reduction catalyst (2) ata time of recovering the NO_(X) absorbing reduction catalyst (2), asdegree of the deterioration, determined by the determination means, ofthe NO_(X) absorbing reduction catalyst (2) becomes higher.
 3. Theexhaust gas cleanup device of the internal combustion engine as claimedin claim 2, wherein an air-fuel ratio in the exhaust gas passage (1)which is upstream of the NO_(X) absorbing reduction catalyst (2) is setso that the density of CO in the exhaust gas passage (1) which isdownstream of the NO_(X) absorbing reduction catalyst (2) is keptconstant, at the time of recovering the NO_(X) absorbing reductioncatalyst (2).
 4. The exhaust gas cleanup device of the internalcombustion engine as claimed in one of claims 1 to 3, which comprises:an exhaust gas flow rate detecting means; an NO_(X) density detectingmeans for detecting density of NO_(X) in an exhaust gas; a temperaturesensor (15) for detecting a temperature of the NO_(X) absorbingreduction catalyst (2); and a calculating means for calculating amountof NO_(X) flowing in the NO_(X) absorbing reduction catalyst (2) perunit time, from an exhaust gas flow rate detected by the exhaust gasflow rate detecting means and from an NO_(X) density detected by theNO_(X) density detecting means, wherein a possible NO_(X) absorbingcapacity of the NO_(X) absorbing reduction catalyst (2) is estimated bythe temperature sensor (15), and wherein the NO_(X) absorbing reductioncatalyst (2) is recovered when an estimated amount of NO_(X) flowing inthe NO_(X) absorbing reduction catalyst (2) reaches a possible absorbingamount.
 5. The exhaust gas cleanup device of the internal combustionengine as claimed in claim 4, wherein the possible NO_(X) absorbingcapacity of the NO_(X) absorbing reduction catalyst (2) which isdeteriorated, is estimated, and wherein an interval for executing therich spike is set in compliance with the possible NO_(X) absorbingcapacity.
 6. The exhaust gas cleanup device of the internal combustionengine as claimed in claim 2 or 3, which comprises a secondary airsupply passage and an oxidation catalyst, both of which are in theexhaust gas passage (1) downstream of the NO_(X) absorbing reductioncatalyst (2), wherein CO having passed through the NO_(X) absorbingreduction catalyst (2) is oxidized in the oxidation catalyst.
 7. Theexhaust gas cleanup device of the internal combustion engine as claimedin claim 1, which comprises a three way catalyst (19) which is in theexhaust gas passage (1) upstream of the NO_(X) absorbing reductioncatalyst (2), wherein the three way catalyst (19) has an oxygenabsorbing function and an oxidizing function.
 8. The exhaust gas cleanupdevice of the internal combustion engine as claimed in claim 7, whereinthe three way catalyst (19) and the NO_(X) absorbing reduction catalyst(2) are unitized integrally, wherein the three way catalyst (19) isupstream of the exhaust gas passage (1).